1. Field of the Invention
This invention relates to cable television descrambling systems. More particularly, the invention relates to a cable television system which relocates the service data channel to prevent unauthorized descrambling of premium and pay-per-view programs by unauthorized subscribers.
2. Description of Related Art
Scrambling and descrambling systems are widely used by CATV system operators to control subscriber access to premium and pay-per-view channels. Signals having video and audio content on certain channels are scrambled prior to transmission from the headend of the CATV system operator and are subsequently descrambled at the home of authorized subscribers. A device, commonly referred to as a descrambler or a settop terminal, is installed in a subscriber's home to restore the scrambled signals if the subscriber is authorized to receive the channel.
Many CATV settop terminals utilize a service data channel and a control data channel to determine whether a settop terminal is authorized to descramble a selected channel. A first authorization code is transmitted over the control data channel, which is dedicated to data transmission and is not associated with any broadcast channels. The control data channel originates at the headend of the CATV system and provides information specific to the CATV system such as data for the video programming guide, settop terminal addressable data and other control information such as the set of codes for the services which the settop terminal is authorized to receive.
The settop terminal separately receives a second authorization code which is transmitted over the service data channel for detection by the settop terminal. This authorized code is inserted by the CATV operator on line 18 of the vertical blanking interval (VBI), which is part of each frame of a video signal transmitted over a broadcast channel. The settop terminal compares the second authorization code sent over the service data channel with the first authorization code detected on the control data channel. If these two codes are identical, the settop terminal is authorized to begin descrambling the received signal.
The integrity of current systems using the aforementioned authorization scheme has been subverted. Illegal or "pirate" settop boxes may be configured to authorize descrambling of a channel regardless of the authorization code received on line 18 on the VBI. Additionally, if the control data channel is compromised, the settop terminal will authorize any code detected on line 18, thereby permitting illegal viewing of a program.
It would be desirable to have a system which prevents access to authorization codes and prohibits unauthorized descrambling of signals having video and audio program content.